I worked at the coal face of a UK computer forensics lab and performed production line forensics - day in day out - welcome to the sausage factory

Tuesday, 7 September 2010

Hiberfil Xpress

Departing on platform 2 .... I seem to have lost my train of thought ..... ever since I started drafting this post I have had to cope with lyrics of Crosby, Stills and Nash's Marrakesh Express floating around in my brain. OK I know I've lost two thirds of my readership already - Crosby Stills and WHO?

This post, once I've overcome a touch of nostalgia, is about the use of compression by Microsoft in the Hiberfil.sys file. From a forensic point of view this fact can be quite important and I have seen reference to this compression in a few of the other forensics blogs as the result of the work of Matthieu Suiche. I also know that functionality exists in Xways to decompress Hiberfil.sys but until now this functionality was absent in Encase.

The reason Microsoft uses compression is to minimise the footprint of Hiberfil.sys. The compression seeks to reduce Hiberfil.sys to about 75% of physical memory size. The presence of this compression can be identified easily - it exists in chunks typically 16 x 4096 bytes in size, each chunk having a header \x81\x81xpress . Not all hiberfil.sys files utilise this compression.

The reason it matters to us can be demonstrated by looking at a fairly common task for us forensicators; finding traces of Windows Live Messenger conversations. In the worst case scenario, when logging is turned off and the user has not saved their conversation, traces of conversations may only be found in memory (or artefacts of memory created on disk). Hiberfil.sys is used to store the contents of memory when the computer concerned is hibernated and therefore potentially may contain Microsoft Notification Protocol messages relating to WLM conversation. A fairly typical grep keyword used to find these traces is \x20PF= . When run over a hiberfil.sys containing xpress compression results may appear similar to the following screenshot:

It can be seen that the message and the surrounding MSNP is a little garbled. This is because this message is within a xpress compressed block. Decompressing the block and viewing the same message results in:

It can be seen that the MSNP and the message is now in plain text. Until now achieving the decompression for Encase users required the use of another tool but I am pleased to report that after discussing this issue with Guidance Software's Simon Key he wrote an enscript for this purpose. The script can decompress all xpress blocks within hiberfil.sys and write them out to a logical evidence file. Alternatively it will decompress each block in turn and then perform a keyword search against it. Blocks containing search hits are written into a logical evidence file. The script is available at GSI's download center.

Finding traces of MSNP is only one use, you can find index.dat contents, Limewire search terms and many other interesting artefacts in Hiberfil.sys - happy searching!